The present invention relates to a system for ultrasonically sensing the level of fuel in a liquid fuel tank. It is considered particularly suitable for but not limited to sensing the level of propane liquid in a propane fuel tank and for sensing the quantity of heating oil in a storage tank.
In North America and throughout the world, there are substantial numbers of propane fuel tanks installed outside residential and commercial premises to provide energy for heating, cooling and cooking. These tanks can range in size from ones which have a relatively small capacity (e.g. 50 gallons) to ones which have a relatively large capacity (e.g. 1000 gallons or more). Often, the propane fuel is sold by a propane distributor to its customers under a contract where the distributor provides the customer with the tank in return for the exclusive right to supply the customer with the fuel.
Typically, propane tanks are filled by the distributor on the basis of estimated usage which is calculated using “degree lapse days” which attempts to estimate usage with climatic data, historical data, and time of years predictions. This method has certain advantages including (1) that there is no infrastructure required to produce the data other than the distributor's existing accounting infrastructure and (2) that any sufficiently large distributor will have a significant amount of historical data from which to draw. But, there are also disadvantages. There can be sufficient granularity in the data that a conservative approach to inventory management requires an average fill per tank of 45% to 50% of total volume instead of a more desirable 80% of total volume, the latter of which obviously allows the distributor to make less frequent trips to the customer with a fuel delivery truck. Further, if a distributor's estimates of a estimated usage are sufficiently in error, there can be a significant number of “out of gas” events which may lead to the loss of a customer. Moreover, without knowing the actual inventory kept in its customer's tanks, a distributor cannot use that data to optimize long term/short term purchasing contracts for propane.
Accordingly, there is a need in the propane industry for a direct method of monitoring customer inventory. In this respect, there are existing fuel tank mounted mechanical gauges which can be installed to provide an approximate indication of inventory. However, the readings they provide are typically quite unreliable (e.g. an error range of plus or minus 10% to 20%). An option that would provide improved accuracy would be to install on each tank a totalizing flow meter like that used by a fuel truck when delivering fuel to a tank, or comparable to that used by utilities that supply electricity, natural gas or water to utility customers. Then, the total volume of fuel removed from the tank then could be recorded and regularly compared with the volume when the tank was filled. However, apart from the cost of installing and maintaining such precision flow meters, the customers of a propane distributor are often located in non-urban areas. The distance between such customers can be too large to justify the repetitive manual collection of usage data.
It is known to sense the level of liquid contained in a tank utilizing sonic measurement techniques; typically sonic or ultrasonic measurements made with the use of a piezoelectric crystal. If the level of liquid is known and the geometry of the tank is known, then the volume of liquid in the tank can be calculated based upon the measured level. However, when the liquid in question is propane and the tank in question is a propane tank, problems arise. One problem is the adequacy of acoustic coupling between the ultrasonic transducer and the fuel tank. If the coupling is poor, then it may not be possible to obtain a measurement, or any measurement made may be unreliable. Another problem is the voltage levels which are generated by the transducer. If relatively high voltages are involved, then regulatory approval may be difficult or impossible to obtain. Dominating any solution to such problems is the question of cost. A solution which enables direct measurement but which requires a costly installation on each fuel tank of each customer is an impractical solution.